The present invention relates to a pressure measuring sensor, and in particular to a pressure measuring sensor for sensing the movement of a diaphragm responsive to a pressure difference between both sides thereof and measuring a pressure or a differential pressure applied to that diaphragm.
In the prior art, such a pressure measuring sensor has a thin diaphragm in the central part thereof, and a gauge resistor for sensing a stress of the diaphragm caused by a pressure applied to the diaphragm is formed on one of the faces of the peripheral part of that diaphragm. No matter which side of the diaphragm a pressure is applied to, the stress applied to the gauge resistor mainly comprises a tensile force because of deflection of the diaphragm caused by the pressure. That is to say, while the pressure is small, a tensile force acts in one direction and a compressive force acts in the other direction. As the pressure is further increased, however, the tensile force acting in one direction becomes further large, and the compressive force acting in the other direction gradually changes into a tensile force. This results in a disadvantage that the output characteristic obtained from the gauge resistor varies according to the direction of the diaphragm deflection.
As the prior art in which the above described disadvantage is mitigated, a pressure measuring sensor as disclosed in JP-A-51-69678 can be mentioned. The pressure measuring sensor has a diaphragm comprising a semiconductor such as silicon. The diaphragm has a central part so formed as to be thick and a peripheral part so formed as to be thin and form a strain causing portion which is deformed in response to a pressure applied to the diaphragm. In this case, the thin part has gauge resistors formed by using the diffusion method or the ion implantation method. In such a pressure measuring sensor, vertical bending stress acts upon the strain causing portion in the direction of pressure application. Therefore, a tensile force acts upon gauge resistors in response to one force whereas a compressive force acts upon gauge resistors in response to the other force. Even if directions of pressure application to the diaphragm are different, therefore, nearly the same output characteristics with respect to the both directions, i.e., nearly symmetrical regular and reverse characteristics can be obtained.
If the strain causing portion is so formed as to be sufficiently thin with the object of measuring a low pressure in a pressure measuring sensor having the above described configuration, however, a tensile force predominantly acts upon gauge resistors in response to pressures in both directions in the same way as the first cited example of the prior art in which the entire diaphragm is so formed as to be thin, resulting in degraded linearity of output characteristics of gauge resistors.
In attempting to reduce the size of a pressure measuring sensor, the strain causing portion also becomes small in the nature of things. For securing an output not less than a predetermined level, the thickness of the strain causing portion in the diaphragm must be made further thinner. In this case, however, the above described drawback appears further largely. In the past, therefore, miniaturization was difficult as a matter of fact.